Frequency selection circuit



July 5, 1949. v. H. vocal.

FREQUENCY SELECTION CIRCUIT Filed Dec. 17, 1946 INVENTOR NZY w/ NB 0 Patented July 5, 1949 UNITED STATES PATENT OFFICE FREQUENCY SELECTION CIRCUIT Vernon H. Vogel, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a

corporation of Iowa 9 Claims.

This invention relates to frequency-selecting systems and more particularly to systems for selectively amplifying one of a series of frequencies closely spaced in the frequency spectrum.

A principal object of the invention relates to an arrangement for generating a series of frequencies in conjunction with a frequency selection network employing a tunable locked-in oscillator for the frequency selection control.

Another object relates to an arrangement for selectively transmitting any desired one of a series of frequencies from a multi-frequency generator, by employing as a link between the generator and the output or load, a free-running but locked-in oscillator, in conjunction with an arrangement for automatically disabling the application of any frequencies to the output when the oscillator departs from its predetermined set frequency.

A feature of the invention relates to a novel frequency filtering network connectable to a multi-frequency source, and embodying a free-running oscillator which can be set to any desired frequency to be passed by the network. A special detecting arrangement is also provided to produce an amplified lock-out voltage when the oscillator departs from its predetermined set frequency.

A further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved single frequency selectin and amplifying network which reduces the number of sharply tuned resonant circuits usually employed for sharp frequency selection.

Other features and advantages .not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing which shows one preferred embodiment of the invention, there is represented by the block I any well-known source of multifrequency signals. The source, for example, may comprise a radio frequency oscillator generator and multi-vibrator combination, to provide any one of a series of radio frequency signals spaced apart in the frequency spectrum approximately 10 kilocycles. The selected frequency from source I is coupled, for example by a radio frequency transformer 2, to the control grid circuit of a free-running oscillator generator 3, of any well- .known type comprising, for example, a grid-controlled vacum tube having an electron-emitting cathode 4, control grid 5, screen rid a, and plate or anode 6. The grid-to-cathode circuit comprises an input inductance I, whichforms the secondary of transformer 2, and a parallel tuning condenser B. A grid leak resistor 9 and grid leak condenser I0 are also connected in the grid circuit as is well" known in oscillators of this type. The portion I I of the secondary winding I is connected between the cathode 4 and the plate 6 in series with the tuned output inductance I2 and its parallel tuning condenser I3, through the D. C. blocking condenser M. The D. C. operating potential for the plate 6 is applied from the positive terminal I5 of a suitable power supply, thence through the primary winding I6 of an audio frequency transformer and through inductance I2 to the plate 6. By well-known feedback action, inductances "I and II in conjunction with the electrodes of tube 3, cause that tube to generate oscillations by feedback action, the frequency of the oscillations, in the absence of applied signals from source I, being determined by the setting of condenser 8. The oscillator 3 is adjusted by condenser 8 to the desired frequency to be passed to the output or load I1, this frequency being some multiple of the fundamental frequency of the multi-vibrator source I.

From the foregoing, it will be seen that the grid 5 is self-excited by the sustained radio frequency oscillations generated by feedback action in the tube 3 and also by radio frequency control signals supplied from source i. By reason of the tuned combination I and 8, the grid circuit of tube 3 in itself offers some attenuation for all those applied frequencies except the frequency to which the oscillator 3 is tuned. Since the oscillator 3 is of the free-running type, it locks-in with the particular frequency from the source I which corresponds to the setting of condenser 8. It will be understood that the source I may be provided with a suitable calibrated dial for selecting at its output a frequency which is not too far removed from the tuned setting of condenser 8 so that by well-known action the oscillator 3 can be forced to oscillate at the desired frequency. This feature is not absolutely necessary since the normal locking-in action of oscillator 3 will cause it to lock-in with the nearest adjacent frequency supplied by source I. In other words, if source I supplies two frequencies one of which is closely adjacent the tuned frequency of oscillator 3, the latter will lock-in at this frequency and not at the other supplied frequency which may be outside this lock-in range. There is thus produced in the output of the oscillator a stable wave which is identical in fre quency with the controlling wave from the source I.

The output inductance I2 may form the primary of a radio frequency coupling transformer I8 tuned by means of condenser I9 to the desired frequency of the wave from source I, and connected to the control grid 20 of a suitable radio frequency amplifier tube 2I of the plural grid type preferably of the type having in addition to the control grid a second grid 22 and a third grid 23, located between the cathode 24 and the anode 25. The second grid 22 may act as a so-called shield grid and for this purpose is connected through a current-limiting resistor 26 to the positive terminal 2! of the I). 0. plate power supply. This power supply is also connected in the conventional manner through the output inductance 28 with its parallel tuning condenser 29 to the plate 25. Likewise, in accordance with the conventional practice, the resistor 26 is provided witha radio frequency by-pass condenser 30. The third grid 23 is connected to one terminal of a bias control resistor M, the opposite terminal of which is grounded. Resistor 3! forms the load resistor of a rectifier tube 32 which is connected across the secondary winding 33 of the audio frequency transformer 34. when the oscillator 3 is not generating sustained oscillations by feedback action at the desired frequency supplied from source I, a D. C. load voltage appears across resistor 3! and biases the grid 23 to plate current cutoff, thus positively preventing any frequencies from reaching the output load I1. In other words, when the oscillator 3 as a result of the setting of condenser 8 has some frequency difference with the applied calibrating signal from source I, the tube 3 acts like a detector and produces audio frequency beat signals in its output circuit which appears across the transformer 34. These audio frequency beats are rectified by tube 32 to produce across resistor 3I a negative bias voltage which is applied over conductor 35 to the cutoff control grid 23. On the other hand, when the oscillator 3 is operating as a self -excited feedback oscillator at the same frequency as the calibrating frequency supplied from source I, there is very little if any voltage developed across resistor 3I with the result that the selected frequency only is passed to the output load I'I.

It will be clear from the foregoing that in addition to providing a high degree of selectivity. the system as disclosed also provides a protective element between the source I and the load II, so as to prevent the application of false frequencies to the load, by disabling the amplifier tube 2 I. It will also be understood that the source I may in the conventional manner, be provided with selecting arrangements so as to select a particular frequency for application to the load I1 in which event the tube 3 and its associated circuits act in the nature of an extremely sharp filter to make sure that only the particular desired frequency is passed to the load II.

While one particular circuit arrangement has been disclosed, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A frequency selective network comprising a master source of frequency, a tunable oscillator generator of the grid-controlled free-running tunable type, means to excite the oscillator with a desired frequency from said source, an amplifier coupled to the output of said oscillator, and means automatically disabling said amplifier in response to a difference in frequency between'the The arrangement is such that l set frequency of said oscillator and the excitation frequency from said source, the last-mentioned means including a detector for producing a cutoff bias for said amplifier when the tuned frequency of said oscillator and the frequency of said master source are substantially different.

2. A frequency selective network comprising a master source of frequency, a tunable grid-controlled oscillator generator of the free-running tunable type, means to tune said oscillator, means also to excite said oscillator with a desired frequency from said master source to cause said oscillator to lock-in at said desired frequency, an amplifier tuned to said desired frequency, and means to disable said amplifier in response to a difference in frequency between the tuned frequency of said oscillator and the applied fre quency from said source, the last-mentioned means including a detector for producing a cutoff bias for said amplifier when the tuned frequency of said oscillator and the frequency of said master source are substantially different.

3. A frequency selective network comprising a master source for generating any desired one of a series of frequencies, a load circuit, and means to pass only a desired frequency to said load, said means comprising a self-excited free-running tunable oscillator arranged to be locked-in at said desired frequency and under control of said source, an amplifier coupled to the output of said oscillator for amplifying at said lock-in frequency, and means to disable said amplifier when said oscillator departs from said locked-in frequency, the last-mentionedmeans including a detector for producing a cutoff bias for said amplifier when the tuned frequency of said oscillator and the frequency of said master source are substantially different.

4. A frequency selective network comprising a multi-frequency master source, a grid-controlled free-running self-excited tunable oscillator having its grid also excited by a selected frequency from said source, an amplifier for amplifying the signals from said oscillator at said selected frequency, and means to bias 'said amplifier substantially to plate current cutoff when the self excitation frequency of said oscillator is different from said selected frequency.

5. A frequency selective network comprising a multi-frequency master source, a grid-controlled free running self excited tunable oscillator, means to apply a desired frequency from said source to lock-in said oscillator at the same frequency, an amplifier for amplifying the output of said oscillator at said desired frequency, a source of plate current cutoff bias for said amplifier, means responsive to a' difference between the normal self-excited tuned frequency of said oscillator and said desired frequency to produce a corresponding beat frequency, and means to rectify said beat frequency to produce said plate current cutoff bias for said amplifier.

6. A frequency selective network comprising a multi-frequency master source, a tunable oscillator of the free-running grid-controlled feedback type having means to tune it to any desired frequency, means toiexcite the grid of said oscillator with a signal from said source at said desired frequency, a beat frequency detector connected to the plate circuit of said oscillator, an amplifier coupled to the output of said oscillator for amplification at said desired frequency, and means controlled by said beat detector for disabling said amplifier when said oscillator is generating at a frequency different from said desired frequency.

7. A frequency selective network comprising in sequence a multi-frequency master source of radio frequency signals, a free-running tunable oscillator generator of the locked-in type having its input circuit connected to said source so as to be locked-in at a desired frequency from said source, a radio frequency amplifier coupled to the output of said oscillator, a beat frequency detector coupled to the said circuit of said oscillator, and means responsive to the rectified output of said detector for biasing said amplifier substantially to plate current cutoff when said oscillator departs from said desired frequency.

8. A frequency selective network comprising a multi-frequency master source of radio frequency signals, an oscillator generator of the locked-in type having its input circuit selectively tunable and also connected to said source for locking said oscillator at a desired frequency from said source, means to cause said oscillator to act as a detector of heat frequencies between the normal set frequency of the oscillator and the said desired frequency from said source, and a rectifier for rectifying said beat frequencies to produce a D. 0. plate current cutoff bias for said amplifier.

9. A frequency selective network according to claim 8 in which said amplifier is of the pentode type, the first grid being coupled to the radio frequency output circuit of said oscillator and the suppressor grid of said pentode being connected to the load circuit of said rectifier.

VERNON H. VOGEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,476,721 Martin Dec. 11, 1923 2,153,789 Van Loon Apr. 11, 1939 2,224,224 Hallam Dec. 10, 1940 2,250,596 Mountjoy July 29, 1941 2,261,64=3 Brown Nov. 4, 1941 

